In the United States and most of the developed world, environmental regulations require sanitary landfills to recover landfill gas (methane) in order to minimize emissions. The recovered landfill gas is generally well purified and transported as pipeline gas, or modestly purified and burned on-site to produce electrical energy via an engine-driven generator or a relatively small gas turbine. The rules and regulations allow the pipeline gas or electricity to enter the appropriate utility distribution channel, thereby providing some compensation to the owners of the landfill and the operators of the landfill gas collection, purification, and generation processes. Even though a useful product, pipeline gas or electrical power, is produced by this recovery process, the concept is regarded as an environmental control, rather than a primary source of energy production.
Landfills are characteristically odorous facilities as the incoming trash is odorous. However, placing the incoming trash in the landfill and covering it with soil does not eliminate the odor; it only minimizes the odor. The real problem with respect to odor is water, which allows aerobic and/or anaerobic decomposition of the landfilled trash, thereby adding to the odor problems. Most importantly, during anaerobic decomposition, sulfate salts such as gypsum from discarded wall board in the landfilled trash, can produce hydrogen sulfide, a particularly odorous material. If more water is present, additional odorous substances are produced. Therefore, general operating procedures encourage minimization of water contact in the trash in order to minimize the overall odor problems at the landfill. Landfills over the past twenty five years have been operated as dry as possible, even though the incoming trash may contain 25 weight percent water.
When a landfill cell is completed (i.e., filled) the contained trash is a large loaf-like mass completely wrapped in a plastic barrier and entombed in many feet of soil. The base of the loaf-like mass includes a leachate collection system used to collect any liquid draining from the contents, while the outer surface prevents entry of moisture from the environment. Despite these measures, the encapsulated trash is still quite wet, perhaps 15 weight percent water on average. Once the cell closure is completed, the internal chemistry starts to operate, producing landfill gas and leading eventually to methane production. Initially the oxygen in the system is consumed via oxidation of the trash, thereby producing carbon dioxide and water, represented approximately as:2-CH2—+3O2=2CO2+2H2O.The gas that is produced is evacuated by the gas collection system and the liquid water is evacuated via the leachate collection system.
When all the oxygen has been consumed, the internal chemistry becomes anaerobic, thereby producing a chemically reduced gas instead of a chemically oxidized gas, represented approximately as:3-CH2—+H2O=2CH4+CO.
The important feature is that the hydrogen in the methane gas is derived from the reduction of water, so, as the availability of water decreases, the methane production decreases, eventually reaching a production level so low that recovery is uneconomic. Since the amount of reducible carbon remaining in the landfill, in general, far exceeds the amount of available and usable water, the entire chemical sequence stops before the maximum methane has been produced, or the maximum conversion of carbon has been achieved.
This observation is not revolutionary, as landfill engineers have known of the water availability limitation for many years. In fact, common practice now includes reapplication of leachate to the top surfaces of the loaf-like mass as a procedure for maintaining the water balance, thereby extending the methane production cycle, and, at the same time, consuming leachate. This technique does improve the overall methane yield, but the majority of the added water simply drains through the compacted trash, following the path of least resistance, becoming leachate once again, with a small percentage undergoing reduction to produce methane. A side benefit of leachate recirculation is that the impurities in the leachate are slowly removed, thereby alleviating the final disposal problem.
A process for maintaining proper moisture content throughout the loaf-like mass would allow optimized methane production, a reduction in the volume of the compacted trash as it would be consumed producing methane, consumption of leachate, the likely water source, and recovery of the landfill air space for reuse, perhaps following landfill mining, a technique used to restore landfill air space by excavation and separation of the contents, yielding soil-like material (compost) and non-biodegradable materials which may be recycled (steel, for instance).
Techniques are currently being developed to overcome these liquid water flow property weaknesses. Waste Management, Inc. has designed a landfill cell configuration incorporating an array of horizontal, perforated pipes used for the injection of water and air, and the extraction of the landfill gases. The Waste Management, Inc. landfill is described in Hater et al., U.S. Pat. No. 6,283,676. Hater et al. U.S. Pat. No. 6,283,676 contains an excellent review of past technology directed at increasing the methane production, and is incorporated by reference in its entirety.
The main objective of this developing technology is air space recovery, and the technique allows degradation to start early in the cell filling process. The initial phase of treatment involves cell hydration using either leachate and/or fresh water, followed by air injection to initiate composting, which generates heat, thereby warming the entire landfill mass. The initial hydration process essentially floods the landfill mass in order to assure maximum hydration. This procedure, of course, requires large volumes of liquid, as the landfill pore volume and other void space must be filled. The excess water remaining at the end of the hydration process drains back into the leachate collection system for either subsequent use or final disposal. After air injection has been completed, the system is chemically deprived of oxygen, allowing anaerobic decomposition to follow. The exit gas then contains methane.
These anaerobic conditions allow the sulfate salts to be reduced, producing small amounts of hydrogen sulfide. The hydrogen sulfide is responsible for at least two problems. First, the hydrogen sulfide must be removed from the extracted gas in order to minimize combustion engine deposits and/or corrosion, and sulfur containing exhaust gas emissions. Second, because hydrogen sulfide is noticeable even at trace levels, even small amounts seeping from the landfill cause odor problems.
Hydrogen sulfide removal from gas and liquid streams is a developed technology, generally involving metal ion catalysis. For more than thirty years, various inventors have patented hydrogen sulfide removal processes. See, for instance, Roberts U.S. Pat. No. 3,622,273, Mancini U.S. Pat. No. 4,011,304, Sibeud U.S. Pat. No. 4,036,942, Lampton U.S. Pat. No. 4,683,076, and Winchester U.S. Pat. No. 6,500,237. There are many others not cited. In general, these removal processes are designed to remove the hydrogen sulfide gas contained in a process stream, for instance, the gas stream exiting from a landfill and being delivered to the gas treatment plant for purification. These process schemes can remove the hydrogen sulfide in the gas streams, thereby reducing or eliminating corrosion problems and combustion exhaust gas emission problems.
Even though these hydrogen sulfide-containing gas streams may contribute to the general landfill odor; they are not responsible for the main sulfide odor problem. The main odor source is fugitive hydrogen sulfide, seeping at very low concentrations from the landfill via an array of pathways. The gas does not just escape from an opening in the landfill's surface. Rather, the concentration is very low but the gas is essentially everywhere. Since the cross sectional area of a landfill is very large, and the hydrogen sulfide concentration is very small, the problem does not lend itself easily to a simple and cost effective control process.